The present application relates to load control methods and apparatus and, more particularly, to novel apparatus and methods for controlling a load resistance, such as a heating element, an incandescent lamp and the like load, to have a desired magnitude, thereby to control filament temperature and the like.
It is often desirable to control the resistance of a load having a non-zero temperature-coefficient load resistance. Such loads include, but are not limited to, radiant heaters, cooking and baking appliances and incandescent lamps. The load may be a low-voltage load operated from a relatively higher voltage A.C. source. For example, there exists a class of low-voltage (24-36 volts) lamps having an improved efficacy due to operation of the lamp filament at a higher temperature than has been traditionally utilized for lamp operation. Because of the higher temperature operation, it is highly desirable to carefully control lamp filament temperature to assure both a controlled light output and reasonable lamp life. As the filament temperature is a function of the lamp resistance (which has a non-zero temperature coefficient) the lamp temperature may be controlled by controlling the load lamp resistance. The lamp may be operated from the commercial A.C. power mains, having a nominal 120 volt r.m.s. value, in residential and commercial operations. Because the voltage desired across the load is lower than the mains voltage, a highly efficient low-voltage power supply is required for energizing the lamp. One such power supply, using a dynamic switched-capacitor configuration, is described and claimed in co-pending application Ser. No. 379,393 filed May 18, 1982 assigned to the assignee of the present invention, and incorporated herein in its entirety by reference. The embodiment of low-voltage incandescent lamp power supply described in detail therein allows for open-loop establishment of the lamp load current (and therefore lamp load voltage, for a particular lamp having a particular resistance at a specified current level, and load power). It is highly desirable to be able to monitor the lamp filament resistance and to provide for closed-loop control of the lamp temperature responsive to variations in such resistance. It is also desirable to provide apparatus and methods for controlling the resistance of other loads having a non-zero temperature coefficient.